Porous materials via freeze-casting of metal salt solutions

ABSTRACT

Disclosed here is a method for making a nanoporous material, comprising aerosolizing a solution comprising at least one metal salt and at least one solvent to obtain an aerosol, freezing the aerosol to obtain a frozen aerosol, and drying the frozen aerosol to obtain a nanoporous metal compound material. Further, the nanoporous metal compound material can be reduced to obtain a nanoporous metal material.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC52-07NA27344 awarded by the U.S. Department of Energy and underGrant No. BRCALL08-PR3-C-2-0006 awarded by the Defense Threat ReductionAgency (DTRA) of the U.S. Department of Defense. The government hascertain rights in this invention.

BACKGROUND

Nanoporous materials (e.g., polymers, metals, inorganic compounds) arethree-dimensional structures that resemble foam. The interconnectedligaments and struts (typically less than 1000 nm in diameter) formcomplex networks that provide porosity of often more than 50%. Thisnano-architecture offers high specific surface areas and ultralowdensity in some cases. While nanoporous compounds such as silica andalumina are mass-produced, nanoporous metals have conventionallyrequired much more complex synthesis strategies (e.g., templatedassembly, dealloying, sol-gel approaches, nanosmelting, super-criticaldrying, or combustion synthesis), making broad deployment economicallyunfeasible. Nonetheless, nanoporous metal foams are compelling materialsas they can maintain good electrical and thermal conductivity whileoffering size-effect-enhanced activity, tunable density and specificsurface area, and novel electro/mechanical behavior (Tappan et al.,Angew. Chem. Int. Ed., 2010, 49:4544-4565). Consequently, nanoporousmetals are being sought for applications such as catalysis, battery andcapacitor electrodes, heat sinks, hydrogen storage, filtration,antimicrobial scaffolds, high-energy density physics experiments, andinks for additive manufacturing of printed batteries and sensors.

Thus, a need exists for improved methods for producing nanoporous metaland metal compound materials.

SUMMARY

Disclosed here is an innovative process for producing nanoporous metaland metal compound materials based on freeze-drying of aerosolized andoptionally pressurized solutions, and subsequent thermal processing.Compared to conventional approaches, the process described herein offershigh yield, high purity, and uniformity. The process can generate bothporous micro-particles and macroscopic monoliths with down to nanometerscale ligaments/struts. The density of the nanoporous material can becontrolled by adjusting the concentration of metal salts in the startingsolution and/or by the compaction of the freeze-dried porous material.

One aspect of the invention described herein relates to a method formaking a nanoporous material, comprising: aerosolizing a solutioncomprising at least one metal salt and at least one solvent to obtain anaerosol; freezing the aerosol to obtain a frozen aerosol; and drying thefrozen aerosol to obtain a nanoporous metal compound material.

In some embodiments, the method further comprises reducing thenanoporous metal compound material to a nanoporous metal material. Insome embodiments, the nanoporous metal material comprises at least about40 wt. % of elemental metal.

In some embodiments, the method further comprises assembling thenanoporous metal compound material into a macroscopic monolith. In someembodiments, the method further comprises assembling the nanoporousmetal material into a macroscopic monolith.

In some embodiments, the solution comprises a silver salt and/or acopper salt. In some embodiments, the solution comprises at least twometal salts. In some embodiments, the solution comprises water as thesolvent.

In some embodiments, the solution is aerosolized by a nebulizer, anozzle, a syringe, and/or a sprayer. In some embodiments, the aerosolhas an average or mean droplet diameter of about 100 microns or less.

In some embodiments, the aerosol is frozen by contacting with liquidnitrogen, a cryogen, a cold surface, and/or a cold gas. In someembodiments, the frozen aerosol is dried in a vacuum chamber. In someembodiments, the frozen aerosol is dried at a temperature of about −90°C. to about 25° C.

In some embodiments, the nanoporous metal compound material isdecomposed and/or reduced by thermal and/or gas treatment. In someembodiments, the nanoporous metal compound material isdecomposed/reduced thermally at a temperature of about 700° C. or less.In some embodiments, the nanoporous metal compound material is reducedby light.

Another aspect of the invention relates to a nanoporous metal compoundmaterial obtained by the method described herein, comprising nanoporousmetal compound particles or foams having an average or mean diameter ofabout 100 microns or less.

In some embodiments, the nanoporous metal compound material comprisesnanoporous metal compound particles or foams which comprise a network ofinterconnected ligaments and struts having an average or mean diameterof about 1000 nm or less.

Another aspect of the invention relates to a nanoporous metal materialobtained by the method described herein, comprising nanoporous metalparticles or foams having an average or mean diameter of about 100microns or less.

In some embodiments, the nanoporous metal material comprises nanoporousmetal particles or foams which comprise a network of interconnectedligaments and struts having an average or mean diameter of about 1000 nmor less.

These and other features, together with the organization and manner ofoperation thereof, will become apparent from the following detaileddescription when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Schematic of an example process for making nanoporous materials.The process yields nanoporous particles with high uniformity andporosity.

FIG. 2: (A) Aerosolized solutions provide an effective route togenerating nearly spherical particles. (B-D) Nanoporous CuSO₄ particlesproduced by freeze-drying. (E-F) Nanoporous Cu/Cu₂O produced by thermalprocessing of (B), which can be further reduced to pure Cu. Scale barsare 5 μm (B, C, E) and 1 μm (D, F).

FIG. 3: Thermogravimetric analysis for the conversion of silver acetate(AgOAc) to Ag metal. Kinetics for SFD (spray freeze-drying) product (ca.30 min) are much faster than for a bulk sample of AgOAc (ca. 120 min).Dashed line indicates point at which 210° C. is reached. Conditions: 10K/min ramp rate from r.t. to 210° C., hold at 210° C., Ar flow 40mL/min.

FIG. 4: SEM (scanning electron microscope) images of time/temperatureinduced coarsening of Ag Foams produced by the SFD process. Conditions:210° C. with varying dwell and cool-down times, N₂ flow 40 mL/min.

FIG. 5: X-ray diffraction spectra for (a) SFD-produced AgOAc and (b)bulk AgOAc reference. The curves are plotted with square root on they-axis to bring out the low intensity peaks. The majority of the peaksobserved match those of silver acetate from the structure database. Peakwidths for the reference sample (b) are sharper than for the SFD samplesuggesting that the reference sample is more ordered or that the domainsize in the SFD sample is smaller.

DETAILED DESCRIPTION

Reference will now be made in detail to some specific embodiments of theinvention contemplated by the inventors for carrying out the invention.Certain examples of these specific embodiments are illustrated in theaccompanying drawings. While the invention is described in conjunctionwith these specific embodiments, it will be understood that it is notintended to limit the invention to the described embodiments. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.Particular example embodiments of the present invention may beimplemented without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention willsometimes be described in singular form for clarity. However, it shouldbe noted that some embodiments include multiple iterations of atechnique or multiple instantiations of a mechanism unless notedotherwise.

Many embodiments of the invention described herein relate to a methodfor making a nanoporous material, comprising aerosolizing a solutioncomprising at least one metal salt and at least one solvent to obtain anaerosol, freezing the aerosol to obtain a frozen aerosol, and drying thefrozen aerosol to obtain a nanoporous metal compound material.

In certain embodiments, the method comprises the following steps: (1)formulating an amenable solution (which is typically a metal salt inaqueous solution but can be any liquid solution with a metal containingsolute) that is liquid at the conditions prior to solidification and isin a solid phase in the cryogen of choice; (2)aerosolizing/atomizing/spraying the solution by, for example, flow theliquid through a nozzle, with a nebulizer, by disruption of a liquidjet, or combinations of the above; (3) freezing the aerosol typically bycontact with a cryogen such as liquid nitrogen or a cold surface; (4)collecting, filtering, and vacuum desiccating the frozen aerosol,optionally using an in-situ temperature controlled-shelf; (5)optionally, assembly of the aerosol particles into macroscopic parts;and (6) optionally, converting the product to a desired state, typicallywith controlled-environment thermal processing (e.g., reduction ofsilver acetate to silver, or reduction of copper sulfate to copperand/or copper oxide) or through surface reaction chemistry with a fluidor vapor that achieves or catalyzes the same.

Formulation of Metal Salt Solution

In some embodiments, the solution comprises salt(s) of at least onemetal selected from Cu, Ag, Au, Fe, Co, Ni, Pd, Pt, Ti, Al, Mg, Li, Pb,Zn, Cr, Mo, W, Ru, Rh, Os, Sm, and Mn. In some embodiments, the solutioncomprises at least one copper salt. The copper salt can be, for example,CuSO₄, Cu(NO₃)₂, Cu(OAc)₂. In some embodiments, the solution comprisesat least one silver salt. The silver salt can be, for example, AgC₂H₃O₂,AgCl, or AgNO₃.

In some embodiments, the solution comprises a metal salt that can bedirectly reduced to a corresponding elemental metal. In someembodiments, the solution comprises a metal salt that can be convertedto a corresponding metal oxide.

In some embodiments, the solution comprises at least two different metalsalts. The two metal salts can comprise the same metal or two differentmetals. When two different metal salts are involved, the processdescribed herein can produce nanoporous alloys and multiphase products.In some embodiments, the solution comprises at least one copper salt andat least one non-copper salt. In some embodiments, the solutioncomprises at least one silver salt and at least one non-silver salt.

In some embodiments, the solution is an unsaturated solution of at leastone metal salt. In some embodiments, the solution is a saturatedsolution of at least one metal salt. In some embodiments, the solutionis a super-saturated solution of at least one metal salt. In someembodiments, the concentration of the metal salt in the solution isabout 50%, or about 60%, or about 65%, or about 70%, or about 75%, orabout 80%, or about 85%, or about 90%, or about 95% of a saturatedsolution. In some embodiments, the solution comprises at least onecopper salt, and the concentration of the copper salt is about 0.5 M. Insome embodiments, the solution comprises at least one silver salt, andthe concentration of the silver salt is about 0.05 M.

A suitable solution should be able to be frozen in available cryogenssuch as liquid nitrogen. Also, the selected metal salt should havereasonable solubility in the solvent chosen.

In some embodiments, the solution comprises water as the solvent. Insome embodiments, the solution comprises at least one organic solvent,such as alcohol, ketones, glycols, kerosene and other aliphatic solventsand carbon tetrachloride and other halogenated solvents. In someembodiments, the solution comprises at least one inorganic solvent, suchas liquid anhydrous ammonia and other solvents with appropriatepolarity, freezing, and evaporation/volatility characteristics. In someembodiments, the solution comprises at least two solvents that aremiscible with each other.

In some embodiments, the solution further comprises at least oneadditive for modifying the assembly of salt crystal structures. Theadditive can be, for example, surfactants, pH modifiers, and surfacebinding species to alter crystal growth kinetics, such as coppercitrate.

Generating the Aerosol

Substantially uniform particle size distributions (e.g., log-normal) canbe obtained using freeze-dried aerosols because the particle dimensionsare established during the vapor phase. Mechanical techniques (e.g.,milling, grinding, micronization) cannot readily reach the nanoscale andcannot readily produce uniform shapes.

Accordingly, in some embodiments, the method described herein comprisesaerosolizing a metal solution by a nebulizer, a nozzle, a syringe,and/or a sprayer.

In some embodiments, the aerosol has an average or mean droplet diameterof about 200 microns or less, or about 100 microns or less, or about 50microns or less, or about 20 microns or less, or about 10 microns orless.

In some embodiments, the aerosol is generated outside a cryogen bath andsprayed onto the cryogen bath, optionally placed next to a nebulizer, anozzle, a syringe, and/or a sprayer. In some embodiments, the aerosol isgenerated by a nebulizer, a nozzle, a syringe, and/or a sprayer immersedin a cryogen bath.

In some embodiments, an ultrasonic nebulizer (e.g., Sonaer 241 PGT)coupled with a gas-driven nozzle are used to generate the aerosol. Thesize of the aerosol droplets, the nozzle position and rate of aerosolflow can be precisely controlled.

In some embodiments, an electronic high-pressure syringe (e.g, Teledyne100 DX) is used to generate the aerosol. This high-pressure syringe canbe used in conjunction with an aerosolizing nozzle with exquisitecontrol. The high-pressure syringe also provides an option to immersethe output nozzle into liquid nitrogen, wherein the injected solutionunder high pressure (>5000 psi) would provide a continuous liquid jetthat turbulently interacts with the cryogen before freezing. During thisturbulent stage, small particles can be formed. This approach does notrequire equilibrium starting solutions and is compatible with both supersaturated solutions and suspensions.

In some embodiments, an ultrasonic nozzle (e.g., Sono-tek 180 kHzNozzle) is used to create reproducible and controlled aerosols of metalsalt solutions. The mean droplet size of the aerosol can be selected byadjusting the ultrasonic frequency. Additionally, nitrogen gas can beused to increase the velocity of the resulting spray and shape theresulting spray, which can alter the freezing profile and mitigatedroplet interaction.

Freezing the Aerosol

In the freezing stage, the solvent undergoes a liquid-solid phasetransition, and the metal salt experiences super-saturation,precipitation, and segregation. By controlling the aerosol size asdiscussed in the foregoing section, radial size-effects can be achieved.

In some embodiments, the aerosol is frozen by contacting a cryogen. Insome embodiments, the aerosol is frozen by contacting liquid nitrogen.In some embodiments, the aerosol is frozen in a cryogen bath. In someembodiments, the aerosol is frozen on a cold surface. In someembodiments, the aerosol is frozen by contacting a cold gas.

Vacuum Desiccation

The frozen product can be collected and optionally filtered, andtransferred to a suitable vacuum chamber for drying by sublimation ofthe solvent (e.g., water).

In some embodiments, the frozen aerosol is dried in a vacuum chamber. Insome embodiments, the frozen aerosol is dried on an in-situ cryo shelfor a temperature controlled shelf. In some embodiments, the frozenaerosol is dried at a temperature of about −90° C. to about 0° C., e.g.,when water is the solvent. In some embodiments, the frozen aerosol isdried at a temperature of about −90° C. to about 25° C. or about −50° C.to about −5° C. The drying temperatures are preferably scaled to thecharacteristics of the chosen solvents and salts.

In some embodiments, the method involves microwave-assisted drying,which may reduce drying times and vacuum requirements.

In some embodiments, the method comprises substantially or totallyremoving the solvent and the cryogen from the frozen aerosol. In someembodiments, the solvent and the cryogen are substantially or totallyremoved from the frozen aerosol by sublimation.

In some embodiments, the freeze-dried nanoporous metal compound materialis a nanoporous assembly of the starting metal salt(s).

In some embodiments, the freeze-dried nanoporous metal compound materialhas a porosity of at least about 30%, or at least about 40%, or at leastabout 50%, or at least about 60%, or at least about 70%, or at leastabout 80%. In some embodiments, the freeze-dried nanoporous metalcompound material has a density of about 1000 mg/cc or less, or about100 mg/cc or less, or about 10 mg/cc or less.

In some embodiments, the freeze-dried nanoporous metal compound materialcomprises nanoporous particles or foams having an average or meandiameter of about 200 microns or less, or about 100 microns or less, orabout 50 microns or less, or about 20 microns or less, or about 10microns or less, or about 5 microns or less.

In some embodiments, the freeze-dried nanoporous metal compound materialcomprises nanoporous particles or foams which comprise a network ofinterconnected ligaments and struts having an average or mean diameterof about 1000 nm or less, or about 500 nm or less, or about 200 nm orless, or about 100 nm or less, or about 50 nm or less, or about 20 nm orless, or about 10 nm or less.

In some embodiments, the freeze-dried nanoporous metal compound materialcomprises substantially homogeneous nanoporous particles or foams. Insome embodiments, at least 30%, or at least 50%, or at least 70%, or atleast 90% of the nanoporous metal compound particles have diameters thatare within about 50-150% of the average or mean diameter of allnanoporous particles in the composition. In some embodiments, at least30%, or at least 50%, or at least 70%, or at least 90% of the nanoporousmetal compound particles have diameters that are within about 70-130% ofthe average or mean diameter of all nanoporous particles in thecomposition. In some embodiments, at least 30%, or at least 50%, or atleast 70%, or at least 90% of the nanoporous metal compound particleshave diameters that are within about 80-120% of the average or meandiameter of all nanoporous particles in the composition.

In some embodiments, at least 30%, or at least 50%, or at least 70%, orat least 90% of the nanoporous metal compound particles have diametersthat are within about 1-5 microns. In some embodiments, at least 30%, orat least 50%, or at least 70%, or at least 90% of the nanoporous metalcompound particles have diameters that are within about 5-10 microns. Insome embodiments, at least 30%, or at least 50%, or at least 70%, or atleast 90% of the nanoporous metal compound particles have diameters thatare within about 10-20 microns. In some embodiments, at least 30%, or atleast 50%, or at least 70%, or at least 90% of the nanoporous metalcompound particles have diameters that are within about 20-30 microns.In some embodiments, at least 30%, or at least 50%, or at least 70%, orat least 90% of the nanoporous metal compound particles have diametersthat are within about 30-50 microns.

Post-Freeze-Drying Processing

The freeze-drying process described in the foregoing sections typicallyproduce nanoporous foams of the starting solute. For example, CuSO₄foams can be produced from CuSO₄(aq), and AgC₂H₃O₂ foams can be producedfrom AgC₂H₃O₂(aq). Such salt foams can be thermally or otherwisedecomposed to elemental metal and/or metal oxide, while maintaining thenanoporous architecture.

Accordingly, in some embodiments, the method further comprises reducingthe nanoporous metal compound material to a nanoporous metal or metaloxide material. In some embodiments, porosity and density of thenanoporous metal compound material are preserved when beingdecomposed/reduced to the nanoporous metal or metal oxide material.

In some embodiments, the nanoporous metal compound material isdecomposed and/or reduced by thermal or gas treatment. In someembodiments, the nanoporous metal compound material is decomposed and/orreduced thermally at a temperature of 800° C. or less, or 700° C. orless, or 600° C. or less, or 500° C. or less, or 400° C. or less, or350° C. or less, or 300° C. or less, or 250° C. or less, or 200° C. orless. The decomposing or reducing environment can comprise, for example,at least one or more of the following gasses: N₂, Ar, CO, H₂, NH₃, CH₄,and H₂S.

In some embodiments, the method comprises reducing copper oxide (whichis converted from a copper salt such as copper sulfate) to copper usingat least one or CO, H₂, NH₃, CH₄, and H₂S. Copper sulfate can be reducedto copper metal in a two-step process via an oxide intermediate.

In some embodiments, the method comprises reducing silver acetate tosilver by thermal treatment. Silver acetate can be thermally decomposedto form metallic silver at about 210° C. This operation can be donewithout severe densification of the product.

In some embodiments, the nanoporous metal compound material is reducedby light, which could mitigate heat-induced coarsening. In oneembodiment, the method comprises reducing silver acetate to silver bylight.

In some embodiments, the nanoporous metal material obtained comprises atleast about 40 wt. %, or at least about 50 wt. %, or at least about 60wt. %, or at least about 70 wt. %, or at least about 80 wt. %, or atleast about 90 wt. %, or at least about 95 wt. %, or at least about 99wt. % of elemental metal. In some embodiments, the nanoporous metalmaterial obtained consists essentially of or consists of elementalmetal.

In some embodiments, the nanoporous metal material obtained has aporosity of at least about 30%, or at least about 40%, or at least about50%, or at least about 60%, or at least about 70%, or at least about80%. In some embodiments, the nanoporous metal material obtained has adensity of about 1000 mg/cc or less, or about 100 mg/cc or less, orabout 10 mg/cc or less.

In some embodiments, the nanoporous metal material comprises nanoporousparticles or foams having an average or mean diameter of about 200microns or less, or about 100 microns or less, or about 50 microns orless, or about 20 microns or less, or about 10 microns or less, or about5 microns or less.

In some embodiments, the nanoporous metal material comprises nanoporousparticles or foams which comprises a network of interconnected ligamentsand struts having an average or mean diameter of about 1000 nm or less,about 500 nm or less, or about 200 nm or less, or about 100 nm or less,or about 50 nm or less, or about 20 nm or less, or about 10 nm or less.

In some embodiments, the nanoporous metal material comprisessubstantially homogeneous nanoporous particles or foams. In someembodiments, at least 30%, or at least 50%, or at least 70%, or at least90% of the nanoporous metal particles have diameters that are withinabout 50-150% of the average or mean diameter of all nanoporous metalparticles in the composition. In some embodiments, at least 30%, or atleast 50%, or at least 70%, or at least 90% of the nanoporous metalparticles have diameters that are within about 70-130% of the average ormean diameter of all nanoporous metal particles in the composition. Insome embodiments, at least 30%, or at least 50%, or at least 70%, or atleast 90% of the nanoporous metal particles have diameters that arewithin about 80-120% of the average or mean diameter of all nanoporousmetal particles in the composition.

Monolith Fabrication

The as-formed particles can be used to produce monolithic, macro-scalestructures of arbitrary dimensions, which can further be machined intoappropriate geometries for a given application. The monoliths can befabricated in arbitrary shapes by spray-casting into forms/molds, whichare removed after the freeze-drying and reduction steps.

Further, electrophoretic deposition (EPD) can be used to fabricategraded density monolith structures. Suspensions of particles withdifferent densities can be used sequentially to build up layers. Inaddition, complex geometries can be obtained through powdermetallurgical strategies including rapid sintering/annealing.

In some embodiments, the method described herein comprises assemblingthe nanoporous metal compound material obtained by freezing drying ofthe aerosol into a macroscopic monolith. In some embodiments, the methoddescribed herein comprises assembling the nanoporous metal materialobtained by reduction into a macroscopic monolith.

Applications

The nanoporous materials described herein have various applications. Forexample, the nanoporous material can serve as catalysts, as electrodesin energy storage devices (e.g., batteries, capacitors), as hydrogenstorage materials, as X-ray sources, as components of advanced inks foradditive manufacturing (e.g., 3D printing), in heat sinks, in filtrationdesalinization, as a desiccant, as antimicrobial/antibacterialmaterials, as bio-scaffolds, and in drug delivery.

WORKING EXAMPLES Example 1—Copper-Containing Nanoporous Material

A. Experimental Procedures for Making Nanoporous Copper from CopperSulfate.

Deionized water was mixed with solid copper sulfate (CuSO₄) to prepare aroom temperature 0.5 M aqueous solution. A carefully cleaned syringepump equipped with a 25 mL glass syringe body was filled with thesolution. The solution was then injected at 8 ml/min into a conicaltipped ultrasonic nozzle (Sono-tek) operating at 180 kHz, 3 W. Thenozzle tip was then immediately positioned approximately 10 cm above acylindrical glass dewar filled with liquid nitrogen. Immersed within theliquid nitrogen, approximately 10 cm below the surface, was a Pyrex, 250mL beaker nested within a stainless steel beaker of the same size(hereafter, the beaker). Approximately 100 mL of solution (4×25 mL) wasaerosolized into the liquid nitrogen. Subsequently, the beaker wasremoved from the liquid nitrogen. Approximately half of the liquidnitrogen within the beaker was poured back into the dewar. The beakerand the remaining content of the beaker (a portion of the frozen aerosoland liquid nitrogen) were placed within a vacuum vessel (base pressure˜30 mTorr). Rapid pumping reduced the internal pressure and induced theliquid nitrogen to change phase into solid nitrogen ice. The nitrogenice sublimed over several minutes. Next, the frozen water sublimed. Theproduct was left to dry in vacuum for approximately 48 hours, afterwhich the system pressure was stable near the base pressure (˜30 mTorr)and the pressure rise upon closing off the pump was consistent withbaseline testing, pre-sample. A white-blue powder was observed withinthe beaker. The beaker was removed from vacuum, preferably withoutambient exposure (e.g., under dry nitrogen), and the contents weretransferred into a quartz crucible. The crucible was placed within asealed tube furnace held at 600° C. for 4 hour, under flowing nitrogen,to decompose the CuSO4 to CuO. The product, black in color, remained inthe furnace at 250° C. for 2 hours, under flowing hydrogen. The furnacewas cooled rapidly. The resulting product is nanoporous copper.

B. Materials Characterization—As shown in FIG. 2, freeze-drying of theaerosolized CuSO4 solution successfully produced nanoporous CuSO4particles (FIG. 2B-2D). The subsequent thermal processing reduced thenanoporous CuSO4 particles to Cu/Cu2O foams (FIG. 2E-2F).

Example 2—Silver-Containing Nanoporous Material

A. Experimental Procedures for Making Nanoporous Silver from SilverAcetate.

Deionized water was mixed with solid silver acetate (AgOAc) to prepare aroom temperature 0.05 M aqueous solution. A carefully cleaned syringepump equipped with a 25 mL glass syringe body was filled with thesolution. The solution was then injected at 8 ml/min into a conicaltipped ultrasonic nozzle (Sono-tek) operating at 180 kHz, 3 W. Thenozzle tip was then immediately positioned approximately 10 cm above acylindrical glass dewar filled with liquid nitrogen. Immersed within theliquid nitrogen, approximately 10 cm below the surface, was a Pyrex, 250mL beaker nested within a stainless steel beaker of the same size(hereafter, the beaker). Approximately 100 mL of solution (4×25 mL) wasaerosolized into the liquid nitrogen. Subsequently, the beaker wasremoved from the liquid nitrogen. Approximately half of the liquidnitrogen within the beaker was poured back into the dewar. The beakerand the remaining content of the beaker (a portion of the frozen aerosoland liquid nitrogen) were placed within a vacuum vessel (base pressure˜30 mTorr). Rapid pumping reduced the internal pressure and induced theliquid nitrogen to change phase into solid nitrogen ice. The nitrogenice sublimed over several minutes. Next, the frozen water sublimed. Theproduct was left to dry in vacuum for approximately 48 hours, afterwhich the system pressure was stable near the base pressure (˜30 mTorr)and the pressure rise upon closing off the pump was consistent withbaseline testing, pre-sample. A white powder was observed within thebeaker. The beaker was removed from vacuum and the contents transferredinto a quartz crucible. The crucible was placed within a sealed tubefurnace held at 210° C. for 1 hour, under flowing nitrogen. The furnacewas cooled rapidly. The resulting product is nanoporous silver.

B. Materials Characterization

As shown in FIG. 3, thermogravimetric analysis (TGA) was used tocharacterize the kinetics of thermal reduction of AgOAc to Ag metal. Itwas found that the SFD foams were reduced to metal much faster than afully dense bulk material sample. The enhanced kinetics can beattributed to reduction in free energy associated with coarsening ofnanostructured ligaments.

As shown in FIG. 4, scanning electron microscopy (SEM) was used tovisually characterize the morphology of the metal-salt foams and reducedmetal foams. Systematic studies were undertaken to understand theeffects of reduction conditions (time/temperature) on the resulting foammorphology. For example, density and ligament size can be controlled inAg metal foams by varying the dwell time (e.g., increasing the annealingtime from 1 to 6 hours at 210° C. as shown in FIG. 2) and coolingconditions in the reduction process.

As shown in FIG. 5, X-Ray diffraction (XRD) was used to determine thecrystallinity of the SFD produced AgOAc material. The peaks detectedconfirm that the material is microcrystalline and not amorphous. Thepeaks are much broader for the SFD-produced material than for areference sample of bulk AgOAc, which suggests that the SFD-producedmaterial is less ordered.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a compound can include multiple compounds unlessthe context clearly dictates otherwise.

As used herein, the terms “substantially,” “substantial,” and “about”are used to describe and account for small variations. When used inconjunction with an event or circumstance, the terms can refer toinstances in which the event or circumstance occurs precisely as well asinstances in which the event or circumstance occurs to a closeapproximation. For example, the terms can refer to less than or equal to±10%, such as less than or equal to ±5%, less than or equal to ±4%, lessthan or equal to ±3%, less than or equal to ±2%, less than or equal to±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or lessthan or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

In the foregoing description, it will be readily apparent to one skilledin the art that varying substitutions and modifications may be made tothe invention disclosed herein without departing from the scope andspirit of the invention. The invention illustratively described hereinsuitably may be practiced in the absence of any element or elements,limitation or limitations, which is not specifically disclosed herein.The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention that in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention. Thus, it should be understood that although the presentinvention has been illustrated by specific embodiments and optionalfeatures, modification and/or variation of the concepts herein disclosedmay be resorted to by those skilled in the art, and that suchmodifications and variations are considered to be within the scopes ofthis invention.

What is claimed is:
 1. A method for making a nanoporous metal material,comprising: aerosolizing a solution comprising at least one metal saltand at least one solvent to obtain an aerosol; freezing the aerosol toobtain a frozen aerosol; drying the frozen aerosol to obtain ananoporous metal compound material; and converting the nanoporous metalcompound material to a nanoporous metal material, wherein the nanoporousmetal material comprises at least about 40 wt. % of elemental metal. 2.The method of claim 1, further comprising assembling the nanoporousmetal material into a macroscopic monolith.
 3. The method of claim 1,wherein the solution comprising a silver salt and/or a copper salt. 4.The method of claim 1, wherein the solution comprising at least twometal salts.
 5. The method of claim 1, wherein the solution comprisingwater as the solvent.
 6. The method of claim 1, wherein the solution isaerosolized by a nebulizer, a nozzle, a syringe, and/or a sprayer. 7.The method of claim 1, wherein the aerosol has an average or meandroplet diameter of 100 microns or less.
 8. The method of claim 1,wherein the aerosol is frozen by contact with liquid nitrogen.
 9. Themethod of claim 1, wherein the frozen aerosol is dried in a vacuumchamber.
 10. The method of claim 1, wherein the frozen aerosol is driedat a temperature of about −90° C. to about 25° C.
 11. The method ofclaim 1, wherein the nanoporous metal compound material is reduced bythermal or gas treatment.
 12. The method of claim 1, wherein thenanoporous metal compound material is reduced thermally at a temperatureof about 600° C. or less.
 13. The method of claim 1, wherein thenanoporous metal compound material is reduced by light.
 14. The methodof claim 1, wherein the nanoporous metal material obtained comprises atleast about 80 wt. % of elemental metal.
 15. The method of claim 1,wherein the nanoporous metal material obtained has a porosity of atleast about 30%.
 16. The method of claim 1, wherein the nanoporous metalmaterial obtained has a porosity of at least about 50%.
 17. The methodof claim 1, wherein the nanoporous metal material obtained has a densityof about 1000 mg/cc or less.
 18. The method of claim 1, wherein thenanoporous metal material obtained has a density of about 100 mg/cc orless.